Electrical transformers exhibit losses both in the windings and in the core, for which reason the heat produced needs to be dissipated. High-power transformers are thus generally cooled using a fluid such as oil. The oils used are dielectric and can ignite above a temperature of the order of 140° C. Since transformers are very expensive, particular attention must be paid to protecting them. An insulation fault first generates a strong electric arc, which prompts action by the electrical protection system, which trips the supply relay of the transformer (circuit breaker). The electric arc also causes consequent dissipation of energy, which generates release of gas from decomposition of the dielectric oil, in particular hydrogen and acetylene. After the gas has been released, the pressure inside the enclosure of the transformer increases very rapidly, hence an often very violent deflagration. The deflagration results in extensive tearing of the mechanical connections in the enclosure (bolts, welds) of the transformer, which brings the said gases into contact with the oxygen in the surrounding air. Since acetylene can spontaneously ignite in the presence of oxygen, combustion immediately starts and causes a fire to spread to other on-site equipments, which may also contain large quantities of combustible products. Explosions are due to short-circuits caused by overloads, voltage surges, progressive deterioration of the insulation, and insufficient oil level, the appearance of water or moisture or the failure of an insulating component. Further it is also observed that there are many instance were fire is started at On Load Tap Changer (OLTC) chamber and then resulting in explosion due to the spread of fire. Thus we see a need to monitor and prevent fire which occurs because of OLTC. Fire protection systems for electrical transformers are known in the prior art, and combustion or fire detectors actuate these. However, these systems are implemented with a significant time lag, when the oil of the transformer is already burning. It is then being necessary to make to with limiting the combustion to the equipment in question, and to prevent the fire from spreading to the neighboring plant. In order to slow down the decomposition of the dielectric fluid due to an electric arc, silicone oils may be used instead of conventional mineral oils. However, explosion of the enclosure of the transformer due to the increase in the internal pressure is delayed only by an extremely short time, of the order of a few milliseconds. This length of time does make it possible to engage means, which can prevent the explosion.
Further it is observed that in most of the fire breakout in the transformer, the source of fire is the OLTC. Hence it is important to first detect and protect OLTC. This would avoid fire breakup to other parts of the transformer. The reason for firebreak more common in OLTC is because of arcing inside the diverter switch.
The document WO-A-97/12379 discloses a method for prevention, protection and/or detection against explosion and/or resulting fire in an electrical transformer provided with an enclosure filled with combustible coolant fluid, by detecting a break in the electrical insulation of the transformer using a pressure sensor, depressurizing the coolant contained in the enclosure, using a valve, and cooling the hot parts of the coolant by injecting a pressurized inert gas into the bottom of the enclosure in order to stir the said coolant and prevent the oxygen from entering the enclosure of the transformer. This method is satisfactory and makes it possible to prevent the enclosure of the transformer from exploding upto some extent. However, the said method does not provide an indication in advance to take corrective measures. Also, by the time the corrective action takes place a significant amount of electrical insulation break down.
An electrical transformer exhibits inherent winding and core losses, generating heat which needs to be dissipated, by natural air cooling or natural oil cooling or forced air cooling or forced oil cooling. Larger electrical transformers are cooled generally using oil, which is a combustible coolant fluid. Larger electrical transformers have a device to detect and release the pressure developed due to expansion of the combustible coolant fluid inside the transformer tank and thereby prevent explosion.
Indian Patent Application IN189089 teaches a method and device for prevention, protection and/or detection of transformer against explosion and/or resulting fire. The patent provides a method of preventing, protecting and/or detecting an electrical transformer from deterioration by way of protecting, preventing and/or detecting said electrical transformer against explosion and/or resulting fire, said electrical transformer having an enclosure filled with a combustible coolant fluid said method comprising the steps of detecting a break in the electrical insulation of the transformer, using a pressure sensor means; partial draining of the coolant contained in the enclosure using a valve and; cooling the hot parts of the coolant by injecting a pressurized inert gas into the bottom of the enclosure in order to stir the coolant and flush the oxygen located in proximity. The said patent particularly refers to a pressure means for prevention, protection and/or detection of transformer against explosion and/or resulting fire, which is different from this invention. The patent fails in clearly mentioning about the nature of the pressure means.
U.S. Pat. No. 6,804,092 discloses a device for prevention, protection and detection against explosion and/or resulting fire of an electrical transformer comprising an enclosure filled with combustible coolant fluid, and a means for decompressing the enclosure of the transformer. The decompression means comprises a rupture element with integrated explosion detector provided with a retention part including first zones which have a reduced thickness in comparison with the rest of the retention part and are capable of tearing without fragmenting when the said element ruptures, and second zones which have reduced thickness in comparison with the rest of the retention part and are capable of folding without tearing when the said element ruptures. The said rupture element is capable of breaking when the pressure inside the enclosure exceeds a predetermined ceiling. The signal from an explosion detector integrated with the rupture disc triggers a cooling system and prevents oxygen from coming into contact with the explosive gases generated by the electric arc in contact with the oil.
U.S. Pat. No. 6,804,092 briefly elucidates regarding “Decompression/Means” wherein a rupture element tears with an increase in pressure in the transformer tank beyond a predetermined ceiling, which is not a foolproof system for detecting and preventing an explosion in an electrical transformer.
Both Prior Art systems discuss pressure developed and subsequent prevention measures through a rapture disc or “Pressure Means”. In both the Prior Art systems namely IN189089 and U.S. Pat. No. 6,804,092 there are inherent disadvantages such as tearing occurring after significant delay from the occurrence the internal electric arc. Therefore it is likely that delay in detection of the pressure developed, explosion and or resulting fire in an electrical transformer take place. None of the prior art teaches the system or method which will detect or prevent occurring of tearing without delay.
Prior Art system disclosed in WO/2007/057916 does not seek protection for OLTC chamber. This invention also has disadvantage that in case of fire or chance of fire in the OLTC gets unnoticed.
Hence it is necessary to overcome the above drawbacks of both prior art and to invent a device and method for prevention, protection and or detection of an electrical transformer from explosion and/or resulting fire which takes preventive and protective measures with least delay.